The present invention generally relates to a noise filter and more particularly, to a noise filter of hermetic type suitable for bringing the whole of a circuit block into hermetic condition.
Generally, in a high frequency circuit, each block of circuits is shielded by covering with the use of a shield case, and a noise filter is used for a power source, input or output lines, etc. to be connected to the block of circuits, such that the circuits are prevented from mutual interference therebetween. Particularly, in the case where high reliability such as wet-proof characteristic or the like is required, a noise filter of hermetic type is employed such that the shielded block of circuits is wholly brought into airtight condition.
For example, as shown in FIG. 2, a noise filter 1 of hermetic type which is employed for the purpose as described above is installed in a shield box 4 accommodating therein a circuit board 3 which loads electronic components 2 such as a transistor, etc., such that the interior of the shield box 4 becomes airtight.
The prior art noise filter of the type referred to above has the construction as shown in FIG. 1.
More specifically, a metal casing 5 opening at opposite ends thereof has an engagement portion 6 formed at the outer perripheral surface of one end portion thereof, and a glass plate 7 secured to the inner face of the other end portion thereof in hermetic condition. A central conductive member 8 having a predetermined length longer than that of the metal casing 5 is fixedly inserted through the glass plate 7. The glass plate 7 of this kind is generally colored and not transparent, or is hardly seen through by the strain inside the glass or the unevenness of the surface of the glass. A first capacitor 9 having a through-hole is provided at one side of the glass plate 7 inside the metal casing 5 such that the central conductive member 8 pierces the capacitor 9. A pair of the electrodes of the first capacitor 9 are fixedly connected to the conductive member 8 and the metal casing 5, respectively, by soldering. At one side of the first capacitor 9 in the metal casing 5 is inserted a ferrite bead 10 with a through-hole through the conductive member 8, which may be fixed by a bonding agent or the like upon necessity. Further, at one side of the ferrite bead 10 remote from the first capacitor 9 in the metal casing 5 is provided a second capacitor 11, through which the central conductive member 8 passes. The second capacitor 11 also has a through-hole. A pair of the electrodes of the second capacitor 11 are respectively fixedly connected to the conductive member 8 and the metal casing 5 by soldering. At the same time, a space formed between the second capacitor 11 and the opening side of the metal casing 5 is filled with an insulative resin 12.
In the noise filter having the construction as described above, it is necessary to confirm by one's eyes the condition how the first and second capacitors 9 and 11 are fixed to the metal casing 5 and the central conductive member 8 by soldering, so as to obtain high reliability.
However, in order to make sure the condition of the soldering by one's eyes in the prior art noise filter, since there is fixedly provided the glass plate 7 at the inner face of the other end portion of the metal casing 5, the confirmation should be carried out twice, namely, first before the ferrite bead 10 is inserted, with the first capacitor 9 being soldered, and second before the insulative resin 12 is filled in, with the second capacitor 11 being soldered, which therefore gives rise to a problem in realization of mass production of the noise filter. Moreover, in the case where the second capacitor 11 is soldered after the first capacitor 9 is soldered in the same manner as described hereinabove, the solder for fixing the first capacitor 9 is melted by the heat generated at the soldering time of the second capacitor 11, resulting in diffusion of the electrodes of the first capacitor 9 during the soldering. Therefore, it is inconvenient that the capacity of the circuit is decreased, with reliability becoming unstable.
For solving the aforementioned problem, it may be proposed that the first and the second capacitors 9 and 11 are soldered simultaneously. However, this proposal is also disadvantageous in reliability since the condition of the soldering of the first capacitor 9 cannot be confirmed by one's eyes because of its construction.